There are known glass melting furnaces, employed in the manufacturing of high-quality glass by heating and melting glass raw materials, that are equipped with a melting vat that heats and melts glass raw materials, a refining vat in which foam is removed from glass that has been melted in the melting vat, and a homogenization vat in which the refined glass is stirred, with the various vats being connected by pipes. For example, Japanese Unexamined Patent Publication (KOKAI) No. 2000-128548 (Reference 1) discloses such a glass melting furnace.
To adequately defoam the glass, the refining vat in the glass melting furnace of Reference 1 is of prescribed shape and gas discharge holes for removing gas generated by the glass melt are provided in the upper portion of the refining vat.
Such methods of manufacturing high-quality glass by passing it through multiple vats such as a melting vat, refining vat, and homogenization vat as set forth above are suited to large-scale production because they permit the continuous manufacturing of glass.
There are broad applications for fluorine-containing glasses such as fluorophosphate glasses in the form of low-dispersion optical glasses and copper-containing near infrared radiation-absorbing glasses. There are also broad applications for fluorine and boron oxide-containing glass—a type of fluorine-containing glass—such as in copper-containing polarizing glass. Thus, fluorine-containing glasses have been put to practical use in recent years, and there is subsequently a need for their large-scale production.
Accordingly, attempts have been made to manufacture fluorine-containing glasses by utilizing the advantages of the above-described method. However, in contrast to other glasses, the manufacturing of fluorine-containing glasses by the above method presents the following problems.
When the glass melt is exposed to gas containing water vapor, the fluorine ions in the glass are replaced by oxygen ions and end up being removed from the glass as HF gas. When the quantity of fluorine ions removed from the glass is large, glass stability is lost or the Abbé number (νd) decreases, precluding use of the glass as optical glass.
To efficiently remove bubbles from the glass, refining must normally be conducted at a temperature exceeding the melting temperature of the glass, and the above-stated loss of fluorine ions and related problems become marked in the refining vat.
Accordingly, the present invention, devised to solve the above-stated problem, has for its object to provide a method of continuously manufacturing high-quality fluorine-containing glass of desired fluorine content, and a method of manufacturing molded glass material from the high-quality fluorine-containing glass continuously manufactured by this method.